Collision avoidance is used to prevent a network node from beginning a transmission on a communications medium while another node is already transmitting on the communications medium. Collision detection is used to halt transmissions when multiple nodes inadvertently transmit on the communications medium simultaneously. Known collision detection methods for bus topology networks compare the data being transmitted with data being simultaneously received at the transmitting node, and report collisions when a mismatch is detected.
Difficulties in collision avoidance and collision detection arise because of propagation delay of signals on the communications medium, and logic delays within the collision detection circuits. These delays result in nodes transmitting simultaneously, and time shifting of the simultaneous transmissions. Time shifting of concurrent transmissions may cause the combined signal of simultaneous dissimilar transmissions to appear identical to the original transmissions of each transmitting node.
Furthermore, asynchronous colliding transmissions may result in the signal transitions of a first transmission occurring during the sampling period of a second transmission. The interpretation of this combined signal by each transmitting node is indeterminate, and may cause the combined signal of simultaneous dissimilar transmissions to appear identical to the original transmission at each concurrently transmitting node. Known collision detection methods relying on data comparison at each transmitting node fail under such circumstances.
In typical CSMA/CD (Carrier Sense Multiple Access with Collision Detection) systems, each node is assigned a unique node address. All transmissions between nodes contain the node address of the transmitting node in the source address field of the transmission, therefore any two concurrent transmissions differ by at least a single bit. However, the format and design of the node address does not account for time shifting of concurrent transmissions, or difficulties in interpretation of asynchronous colliding transmissions. The node address format defines two sub-fields: the company code, and the node specific field. Each sub-field is 24 bits in length. The company code field contains a value assigned to the node's manufacturer. For example, all nodes manufactured by Digital Equipment Corporation contain the value 08-00-2b in the company code sub-field of the source address field. When communications systems include many nodes from the same manufacturer, the unique portion of the source address field is only 24 bits out of the 48 bit source address field. In this 24 bit sub-field, there is at least a one bit difference between any two simultaneous data transmissions. Time shifting of concurrent transmissions, or problems with interpretation of asynchronous colliding transmissions may cause this differentiating bit to be ineffective, and as a result, collisions may go undetected.
"Draft Supplement to ANSI/IEEE Std. 802.3-1990, Type 10Base-F" ("10Base-F"), published by IEEE Stds., 445 Hoes Lane, Piscataway, N.J. 08854, 1991, at page 16-4, all disclosures of which are herein incorporated by reference, assigns a second unique value to each node in the communications system, included as the first 32 bits of every transmission. "10Base-F" is IEEE terminology for a CSMA/CD implementation, where the first two digits indicate a 10 megabit per second data rate, the next four letters indicate baseband signalling, and the final letter indicates that the communications medium is fiber optic cable. The unique value field added by 10Base-F requires costly additional circuitry in each node, not included in standard CSMA/CD circuits. To process the 10Base-F unique value field, nodes must include logic to insert the unique value field into every transmission, compare the transmitted unique value field with the received signal, and remove received unique value fields before the transmission is passed to standard CSMA/CD circuitry.
It is desirable to have an economical collision detection mechanism which reliably detects collisions in the presence of asynchronous simultaneous transmissions transitioning during the sampling period of the station, and which reliably detects collisions despite time shifting of concurrent transmissions caused by the propagation delay of the communications medium, or logic delays within the collision detection circuits, that is compatible with standard CSMA/CD circuits, and does not require costly additional circuitry.